not_equal.R

not_equal <- function(a, b){
  ifelse(a != b | is.na(a) | is.na(b),
            ifelse(is.na(a) & is.na(b),
                   NA,
                   TRUE),
            FALSE)
}

not_equal("A", "A")
#> [1] FALSE

not_equal("A", NA)
#> [1] TRUE

not_equal(NA, "A")
#> [1] TRUE

not_equal(NA, NA)
#> [1] NA